The picornaviruses are a family of small positive sense single stranded RNA viruses that cause a wide range of diseases at an annual cost well into the hundreds of million dollars. Members include acute hepatitis A virus, the heart disease causing coxsackie B3 virus, rhinoviruses that cause more than half the occurrences of the common cold, and the paralyzing poliovirus. These viruses share a life cycle where RNA replication and viral assembly occurs in large membrane anchored replication complexes assembled on the surfaces of vesicles derived from the endoplasmic reticulum. The replication process is driven by a virally encoded RNA dependent RNA polymerase (3Dpol) that is responsible for the synthesis of all viral RNA. This research project is focused on the structure and assembly of the viral replication centers and on structure-function studies of the viral polymerases to understand mechanisms that control of elongation rates and replication fidelity. We have solved multiple crystal structures of 3Dpol as elongation complexes with bound RNA and these structures revealed a novel mechanism by which RdRPs close their active sites for catalysis in response to nucleotide binding. In this upcoming project period we will use biophysical and structural biology methods to further our mechanistic understanding of RNA translocation by 3Dpol and to solve the structures of viral initiation and uridylylation complexes. We will also use single molecule microscopy and particle tracking methods to directly visualize the in vitro assembly of viral replication complexes onto supported lipid bilayers.